Long-Term Evolution (LTE) systems, specified by the 3rd Generation Partnership Project (3GPP), use Orthogonal Frequency Division Multiplexing (OFDM) for downlink transmissions and discrete-Fourier-transform-spread (DFT-spread) OFDM for uplink transmissions. The basic LTE downlink physical resource can thus be viewed as elements in a time-frequency grid. This time-frequency grid is illustrated in FIG. 1, where each resource element 14 corresponds to a single OFDM subcarrier 12 and a single OFDM symbol interval. The OFDM subcarriers 12 in an LTE signal are spaced at 15 kHz; each OFDM symbol 16 comprises an introductory cyclic prefix 17.
In the time domain, LTE downlink transmissions are organized into radio frames of ten milliseconds, each radio frame consisting of ten one-millisecond subframes. This is illustrated in FIG. 2. Generally, resource allocations in LTE are defined in terms of resource blocks, where a resource block corresponds to one slot (0.5 milliseconds, or one-half of a subframe) in the time domain and twelve contiguous subcarriers in the frequency domain. Resource blocks are individually numbered in the frequency domain, starting with resource block number 0, from one end of the system bandwidth to the other.
Downlink transmissions in LTE are dynamically scheduled, in that the base station (known as the evolved-Node B, or eNodeB, in LTE terminology) transmits control information, in each subframe, indicating which mobile stations are scheduled to receive data in the current downlink subframe and further indicating which resource blocks are used for a given mobile station's data. This control signaling is typically transmitted in the first one, two, three, or four OFDM symbols in each subframe. A portion of a downlink OFDM subframe for a system using three OFDM symbols for the control region is illustrated in FIG. 3. FIG. 3 also illustrates that reference symbols, used by the mobile station for channel estimation, channel quality measurements, and cell search and acquisition procedures, are dispersed at various intervals throughout the downlink time-frequency grid.
LTE uses an error control technique known as hybrid-automatic-repeat-request (hybrid-ARQ, or HARQ) for detecting and correcting transmission errors at the medium access control (MAC) protocol layer. Thus, after receiving downlink data in a given subframe, a mobile station attempts to decode it and reports to the base station whether or not the decoding was successful, using an acknowledgement message (ACK) or negative acknowledgement message (NACK), respectively. In the event of an unsuccessful decoding attempt, indicated by receipt of a NACK, the base station can retransmit the erroneous data.
Uplink control signaling from a mobile station to the base station includes HARQ acknowledgements for received downlink data, terminal reports related to the downlink channel conditions (used by the base station as assistance for the downlink scheduling), and scheduling requests, which indicate that the mobile station needs uplink resources for uplink data transmissions. To transmit data in the uplink, a mobile terminal generally must first be assigned an uplink resource for data transmission, on the Physical Uplink Shared Channel (PUSCH). In contrast to a resource assignment in downlink, a given allocation of uplink time-frequency resources must always be limited to a single contiguous range of resource blocks; this is necessary to maintain desired signal properties for the uplink transmissions. Thus, a given user's uplink resource allocation is uninterrupted in the frequency domain by any other user's allocation, as illustrated in FIG. 4.
In an uplink transmission, the middle SC-FDMA symbol in each slot is used to transmit a reference signal. This is shown in FIG. 4, where the fourth (of seven) OFDM symbols of each slot is used to transmit a reference signal. If the mobile station has been assigned an uplink resource for data transmission, and at the same time instance has control information to transmit, it will transmit the control information along with the data on PUSCH. Two types of reference signals are supported on the LTE uplink: a demodulation reference signal, which is associated with the transmission of uplink data and/or control signaling; and a sounding reference signal, which is not associated with uplink data transmission and is used mainly for channel quality determination if channel dependent scheduling is used.
The demodulation reference signal is generated as the product of a base sequence and a reference signal sequence index, in the frequency domain. The base sequence is constructed from so-called Zadoff-Chu sequences, which have good correlation properties. Specifically, the autocorrelation for a Zadoff-Chu sequence is zero for non-zero delays, and the cross-correlation between different sequences is low. For low bandwidth allocations, computer-generated sequences are used instead, to increase the number of available sequences.
Each of several reference signal sequence indexes, called cyclic shifts in LTE, may be applied to a given base sequence to generate several reference signals that are orthogonal to each other. Based on the cell identity for the serving LTE base station, a random base sequence and sequence shift is selected for each slot, to avoid the use of the same reference signal in two adjacent cells.
When multi-user multiple-input multiple-output (MU-MIMO) transmission is used in the uplink, two or more mobile stations in a given cell may transmit using the same or overlapping frequency resources. In this case, the reference signals for the different users will have different cyclic shifts. As a result, the reference signals are orthogonal, and channel estimation can be performed separately for each user. The particular cyclic shift that a given mobile station should use in generating the reference signal is signaled to the mobile station in the uplink grant sent to the mobile station. This signaled cyclic shift is then added to the random cyclic shift based on the cell identity.
In LTE, as in any communication system, a mobile station may need to initiate a data transfer to the network (via the eNodeB) without already having been assigned uplink resources. To handle this, a random access procedure is available where a mobile station that does not have a dedicated uplink resource may nevertheless transmit a signal to the base station. The first message of this procedure is typically transmitted on a special resource reserved for random access, i.e., a physical random access channel (PRACH). As shown in FIG. 5, the LTE PRACH is a pre-determined group 52 of uplink time-frequency resources, appearing in each frame. In the illustrated scheme, the PRACH 52 comprises six contiguous resource blocks in the frequency domain, over a single one-millisecond subframe. The specific resources available for PRACH transmission are configurable, and are identified to the mobile stations as part of the broadcasted system information (or as part of dedicated radio resource control signaling in the event of handover, for example). In LTE, the random access procedure can be used for a number of different reasons. These reasons include: initial access, such as for mobile stations in idle (LTE_IDLE) or detached (LTE_DETACHED) states; incoming handover; resynchronization of a mobile station; transmission of a scheduling request, such as for a mobile station that is not allocated any other resource for contacting the base station or that has sent the base station a maximum allowed number of scheduling requests without any response from the base station.
The contention-based random access procedure specified in LTE is illustrated in FIG. 6. The mobile station 60 (commonly referred to as a UE, or user equipment, in LTE standards) first receives system information for the random access procedures from the LTE Radio Access Network (RAN) 62, as shown at step 63. Later, mobile station 60 initiates the random access procedure by randomly selecting one of several preambles available for contention-based random access, and then transmitting the selected random access preamble on the physical random access channel (PRACH) to eNodeB in RAN 62, as shown at 64. RAN 62 acknowledges any preamble it detects by transmitting a random access response (MSG2), as shown at 65; this random access response includes an initial grant of resources to be used on the uplink shared channel, a temporary C-RNTI, and a time alignment (TA) update based on the timing offset of the preamble measured by the eNodeB.
After receiving the random access response, UE 60 uses the resources specified in the uplink grant to transmit a message (MSG3) that in part is used to trigger the establishment of radio resource control and in part to uniquely identify the UE 60 on the common channels of the cell. The transmission of this message is shown at 66. (Those skilled in the art will note that MSG3 is transmitted on scheduled resources on the transport channel UL-SCH, via physical channel PUSCH, not on PRACH.) The timing alignment command provided in the random access response is applied in the uplink transmission of MSG3.
The procedure ends with RAN 62 resolving any preamble contention that may have occurred in the event that multiple mobile stations transmitted the same random access preamble at the same time. This can occasionally occur, since each mobile station randomly selects when to transmit and which preamble to use. If multiple mobile stations select the same preamble for the transmission on RACH, there will be contention between these mobile stations—this contention is resolved through a contention resolution message (MSG4) transmitted by the RAN 62, as shown at 67. FIG. 7 illustrates a scenario where contention on the RACH at base station 72 occurs because two mobile stations, UE1 and UE2, each transmit the same preamble, p5, at the same time. A third mobile station, UE3, also transmits a random access preamble at the same time, but since it transmits with a different preamble, p1, there is no contention between UE3 and the other mobile stations.